1. Field of the Invention
The invention relates to an electric power steering system including a motor that applies assist force to a steering system.
2. Description of Related Art
Japanese Patent Application Publication No. 2004-66999 describes an electric power steering system that calculates a motor rotational angular velocity ω using Equation A shown below. ω=(Vm−R×IM)/Ke  Equation A
In Equation A above, Vm represents the voltage between terminals of a motor (inter-terminal voltage), R represents the resistance of the motor, Im represents the current of the motor, and Ke is an induced voltage coefficient (V·s/rad). The resistance of the motor is actually measured and stored in advance.
The actual resistance of a motor changes under the influence of the motor ambient temperature. The motor resistance varies from motor to motor. Further, the motor resistance changes due to repeated use over the years.
The electric power steering system described in Japanese Patent Application Publication No. 2004-66999, however, does not address this issue. Therefore, there is a possibility of a large difference between the motor resistance actually measured and stored in advance and the actual motor resistance. Such a large motor resistance difference causes a large difference between the motor rotational angular velocity calculated based on the motor resistance and the actual motor rotational angular velocity.